1. Field of the Invention
The present invention is directed to a method for routing message cells from connections having respectively different priorities particularly in the reception of message cells from low-priority connections, such as non-real time (NRT) traffic, of the type wherein redundant message cells are respectively transmitted via redundant transmission paths, and only one of the received redundant message cells is selected for further processing.
2. Description of the Prior Art
In ATM (Asynchronous Transfer Mode) transmission, the sequence of message cells belonging to a virtual connection should be maintained. In order to meet a prescribed loss probability, parallel routing or forwarding, via through-connections along redundant transmission paths may be necessary. Among the redundantly transmitted message cells, only one message cell is selected at the reception location as the next message expected in the sequence. To designate the sequence, each message cell carries a series number, allocated to it in a cyclically progressing fashion in a field for the series number. The individually distinguishable series numbers of a virtual connection define a message cell sequence cycle.
The redundant transmission paths can each include a statistical multiplexer unit which is intended to form the traffic of low-priority connections during the activity of large buffer memories in such a way that the total traffic loads the output line optimally, i.e. to nearly 100 percent.
The statistical multiplexer units of the coupling field slices operate independently of each other, so that message cell streams with different contents can arise.
If a simple recombination algorithm is applied at the receiving location at the end of the redundant transmission paths, then the message cells which were rejected on one transmission path for reasons of overload are replaced by redundant message cells transmitted on another transmission path, which leads to a load of greater than 100% and thus to an overflow of the RPC buffer at the exit of the output unit.
A selection identification character could be entered in the receiving unit for the connections of lower priority, this selection identification character determining the transmission path by which message cells of the relevant connection are exclusively received for further processing. Given the failure of a transmission path, the board controller would have to rewrite all the affected entries which would last several seconds, given up to 8192 connections, and which would have to be followed by a message cell sequence cycle in order to avoid an undesired mixing of the message cell streams of the low-priority connections of the transmission paths participating in the switchover. During such a cycle the transmission paths participating in the switchover do not receive any message cells for further processing.